This invention relates generally to gas turbine engine bearing sumps and more particularly to control of oil flow in bearing sumps.
A gas turbine engine includes one or more shafts which are mounted for rotation in several bearings, usually of the rolling-element type. The bearings are enclosed in enclosures called “sumps” which are pressurized and provided with an oil flow for lubrication and cooling. In most cases one of the boundaries of the sump will be a dynamic seal between a rotating component of the engine and the engine's stationary structure.
Many dynamic seals, such as carbon seals, require secondary seals to prevent oil leakage past the primary sealing surface. A device called a “windback” comprising a helical thread and mating rotating surface is frequently used. The windage caused by the rotating surface pushes the oil mist away from the interface, causing any oil accumulated within the helical thread to be driven through the thread groove back into the sealed cavity. The axial component of windage generated by the air shearing acts as a driving force to keep oil mist away. The tangential component of windage pushes oil collected at the bottom of helical thread back into sealed cavity. Windage is a secondary effect of shaft rotation and its effectiveness strongly depends on shaft speed and the radial gap between rotating and stationary parts.
In a prior art windback, the grooves between the teeth are at the same diameter; there are no axial or tangential angles to facilitate oil drainage. The pitch of the thread is relatively small compared to the diameter, therefore, the axial windage effect is limited. Furthermore, oil collected at the thread root has to travel through the total length of the thread circumference. Oil collected must overcome gravity to return back to oil-wetted cavity if the shaft axis is horizontal. Under conditions where the windage is not adequate to drive oil completely around circumference of the thread and back to the oil-wetted cavity, oil leakage might occur. Windback effectiveness is usually difficult to predict. If oil/air mist passes the secondary seal, performance of the primary seal is jeopardized.